Thyrotoxicosis is a hypermetabolic state in which there is increased utilization of thiamine. Thiamine deficiency has been observed in association with hyperthyroidism. Several studies documented that thiamine treatment could improve signs and symptoms of congestive heart failure, or even improve left ventricular ejection fraction in patients without thyrotoxicosis. This pilot study aims to evaluate prevalence of thiamine deficiency and assess improvement of cardiovascular function after receiving thiamine supplement in thyrotoxic patients.